In the beginning of the 19th century, William Wollaston and Joseph Fraunhofer developed the basics of modern spectral analysis of light by measuring the sun's electromagnetic radiation spectrum for the first time. Since then, spectroscopy has become an essential tool for scientific research, including analytical chemistry, biochemical sensing, material analysis, optical communication, and medical applications. In a typical commercial spectrometer, a dispersive prism or a grating projects spectral components of light onto a detector array. Since the optical path length limits the spectral resolution, high-performance grating spectrometers are inevitably bulky and expensive.
In recent years, there has been a strong effort to develop compact and monolithic spectrometers. In addition to miniature grating spectrometers, resonant structures such as nanocavities, have been employed to separate spectral components into unique detectors. This research has produced millimeter-scale spectrometers with high resolving powers of Q>104, as quantified by the quality factor Q=ω0/δω, where δω is the spectral resolution at frequency ω0. Another class of spectrometers relies on imaging the speckle patterns of a photonic bandgap fiber bundle or a multimode optical fiber. However, these spectrometers are centimeters to meters long, and the long length makes them susceptible to environmental fluctuations. A recently demonstrated silicon-based spectrometer measures the multimode transmission profile from a disordered photonic crystal structure to achieve a Q comparable to typical grating spectrometers, but requires careful coupling to a sub-wavelength input waveguide and has a limited bandwidth from 1500 nm to 1525 nm. A high-resolution Fourier Transform spectrometer has also been demonstrated on-chip, but its narrow free spectral range limits its operation bandwidth to 0.75 nm at 1550 nm.